The present invention relates to generally a self-scanning light-emitting device, particularly to a self-scanning light-emitting device in which the number of bonding pads can be decreased.
A light-emitting device in which a plurality of light-emitting elements are arrayed on the same substrate is utilized as a light source of a printer, in combination with a driver circuit. The inventors of the present invention have interested in a three-terminal light-emitting thyristor having a pnpn-structure as an element of the light-emitting device, and have already filed several patent applications (see Japanese Patent Publication Nos. 1-238962, 2-14584, 2-92650, and 2-92651.) These publications have disclosed that a self-scanning function for light-emitting elements may be implemented, and further have disclosed that such self-scanning light-emitting device has a simple and compact structure for a light source of a printer, and has smaller arranging pitch of thyristors.
The inventors have further provided a self-scanning light-emitting device having such structure that an array of light-emitting thyristors having a transfer function is separated from an array of light-emitting thyristor having a write function (see Japanese Patent Publication No. 2-263668.)
Referring to FIG. 1, there is shown an equivalent circuit diagram of a conventional self-scanning light-emitting device. This self-scanning light-emitting device is a type of two-phase driving device. In the figure, reference characters T1, T2, T3 . . . designate light-emitting elements, D1, D2, D3 . . . coupling diodes, R1, R2, R3 load resistors, respectively, the light-emitting elements being consisted of three-terminal light-emitting thyristors. All of the cathodes of the light-emitting elements are connected to the ground, the anodes of odd-numbered light-emitting elements to a clock pulse xcfx861 line 11, the anode of even-numbered light-emitting elements to a clock pulse xcfx862 line 12, respectively. Each gate of the light-emitting elements is connected to a power supply voltage xcfx86GK line 14 via respective load resistor R1, R2, R3 . . . . The gate electrodes of neighboring light-emitting elements are connected to each other via respective coupling diodes D1, D2, D3 . . . . Lines 11, 12 and 14 are derived outward via bonding pads 21, 22 and 24, respectively. The gate of the light-emitting element T1 is connected to the bonding pad 23 for a start pulse xcfx86s. In the figure, reference numeral 10 shows a chip for the integrated self-scanning light-emitting device.
Bonding pads 21, 22 and 23 are connected to output terminals 41 (xcfx861), 42(xcfx862) and 43 (xcfx86s) of a driver circuit 40 via exterior current limiting resistors 51, 52 and 53, respectively, and the bonding pad 24 is directly connected to a output terminal 44 (xcfx86GK) of the driver circuit 40.
Referring to FIG. 2, there is shown the timing of driving pulses xcfx861, xcfx862, xcfx86GK and xcfx86s from the driver circuit 40. The levels of each pulse include High level and Low level, Low level being equal to a cathode potential, i.e. a ground potential.
In FIG. 2, L (T1), L (T2), L (T3) . . . show the state of the light emission of the element T1, T2, T3 . . . , the element being emitting state, i.e. on-state at the timing of a shaded area.
The timing diagram of FIG. 2 is illustrated with divided three modes, i.e. MODE-1 (standby mode), MODE-2 (transition mode), and MODE-3 (transfer mode). In the standby mode (MODE-1), all of the light-emitting elements are off-state with xcfx861, xcfx862, xcfx86GK and xcfx86s being Low level. Transition mode (MODE-2) has a time duration during which the power supply voltage pulse xcfx86GK is required to be driven to High level. In the transfer mode (MODE-3), the light-emitting element T1 is turned on when the clock pulse xcfx861 is driven to High level during the start pulse xcfx86s is at Low level. The start pulse xcfx86s is turned to High level just after the element T1 is turned on. After the element T1 is turned on, the on-state of the elements is transferred by means of two-phase clock pulses xcfx861 and xcfx862.
According to the structure of this conventional self-scanning light-emitting device, four bonding pads 21 (xcfx861), 22 (xcfx86), 23 (xcfx86s) and 24 (xcfx86GK) are required in a chip due to the wiring to the driver circuit, consequently it is difficult to make such a chip small.
The object of the present invention is to provide a self-scanning light-emitting device in which the number of bonding pads in a chip may be decreased to 2 or 3.
According to the present invention, the number of pads in a chip may be decreased in a self-scanning light-emitting device comprising an array of a plurality of three-terminal light-emitting elements linearly arranged each having a control electrode for controlling threshold voltage or current; electrical means having unidirectional characteristic to voltage or current for connecting the control electrodes of neighboring light-emitting elements to each other; two clock pulse lines for applying two-phase clock pulses alternately to one of two terminals except the control electrode of each light-emitting element, one phase clock pulse of the two-phase clock pulses causing the threshold voltage or current of the light-emitting elements in the vicinity of a turned-on light-emitting element to vary via the electrical means, and the other phase clock pulse of the two-phase clock pulses causing the light-emitting element neighbored to the turned-on light-emitting element to turn on; and a power supply line connected to each of the control electrodes of the light-emitting elements via a load resistor, respectively.
In order to realize this, the following approaches may be adopted.
(1) The resistance of the load resistor connected to the light-emitting element to be turned on at first is selected to be smaller than that of other resistors. As a result, the bonding pad for a start pulse may be omitted.
(2) A diode or resistor is connected between one of the two clock pulse lines and the control electrode of the light-emitting element to be turned on at first. As a result, the bonding pad for a start pulse may be omitted.
(3) A logical OR circuit consisting of a diode-diode logic is connected between the two clock pulse lines and the power supply line. As a result, the bonding pad for the power supply pulse may be omitted.
(4) A logical OR circuit consisting of a diode-diode logic is connected between the two clock pulse lines and the power supply line, and a diode or resistor is connected between one of the two clock pulse lines and the control electrode of the light-emitting element to be turned on at first. As a result, the bonding pads for the start pulse and the power supply pulse may be omitted.
Also, the present invention is applicable to a type of self-scanning light-emitting device wherein transfer and light emission functions are separated. This type of device comprises an array of a plurality of three-terminal transfer elements linearly arranged each having a control electrode for controlling threshold voltage or current; electrical means having unidirectional characteristic to voltage or current for connecting the control electrodes of neighboring transfer elements to each other; two clock pulse lines for applying two-phase clock pulses alternately to one of two terminals except the control electrode of each transfer element, one phase clock pulse of the two-phase clock pulses causing the threshold voltage or current of the transfer elements in the vicinity of a turned-on transfer element to vary via the electrical means, and the other phase clock pulse of the two-phase clock pulses causing the transfer element neighbored to the turned-on transfer element to turn on; a power supply line connected to each of the control electrodes of the transfer elements via a load resistor, respectively; an array of a plurality of three-terminal light-emitting elements linearly arranged each having a control electrode for controlling threshold voltage or current, each control electrode of the light-emitting elements being connected to corresponding control electrode of the transfer elements; and a write signal line for applying a write signal to one of two terminals except the control electrode of the light-emitting element.
In this self-scanning light-emitting device, the number of the bonding pads may be decreased by applying the approaches (1)-(4) to the part of a transfer function.